Ablation of myocardial tissue is well known as a treatment for cardiac arrhythmias. In radio-frequency (RF) ablation, for example, a catheter is inserted into the heart and brought into contact with tissue at a target location. RF energy is then applied through an electrode on the catheter in order to create a lesion for the purpose of breaking arrhythmogenic current paths in the tissue. Recently, circumferential ablation of the ostia of the pulmonary veins has gained acceptance as a treatment for atrial arrhythmias, and particularly for atrial fibrillation. For example, U.S. Pat. No. 6,064,902, whose disclosure is incorporated herein by reference, describes a catheter for ablating tissue on the inner wall of a blood vessel, such as a pulmonary vein.
Some medical imaging systems use catheters that are fitted with ultrasound transducers. For example, U.S. Pat. No. 5,076,278, whose disclosure is incorporated herein by reference, describes an ultrasonic transducer for use in locating devices. The transducer has a curved surface, and is an annular member in shape. An elongated catheter or other device passes through the opening of the annular transducer. As another example, U.S. Pat. No. 5,081,993, whose disclosure is incorporated herein by reference, describes an ultrasonic probe that is inserted into a human organ. In some embodiments, the probe includes an annular ultrasonic transducer assembly.